Publication Date:
2014-03-14
Description:
Fossil fuel combustion has increased atmospheric CO 2 by ≈ 115 µmol mol -1 since 1750, and decreased its carbon isotope composition (δ 13 C) by 1.7-2 ‰ (the 13 C Suess effect). Because carbon is stored in the terrestrial biosphere for decades and longer, the δ 13 C of CO 2 released by terrestrial ecosystems is expected to differ from the δ 13 C of CO 2 assimilated by land plants during photosynthesis. This isotopic difference between land-atmosphere respiration (δ R ) and photosynthetic assimilation (δ A ) fluxes gives rise to the 13 C land disequilibrium (D). Contemporary understanding suggests that over annual and longer time scales, D is determined primarily by the Suess effect, and thus D is generally positive (δ R 〉 δ A ). A seven-year record of biosphere-atmosphere carbon exchange was used to evaluate the seasonality of δ A and δ R , and the 13 C land disequilibrium, in a subalpine conifer forest. A novel isotopic mixing model was employed to determine the δ 13 C of net land-atmosphere exchange during day and night, and combined with tower-based flux observations to assess δ A and δ R . The disequilibrium varied seasonally, and when flux-weighted was opposite in sign than expected from the Suess effect (D = -0.75 ± 0.21 ‰ or -0.88 ± 0.10 ‰ depending on method). Seasonality in D appeared to be driven by photosynthetic discrimination (Δ canopy ) responding to environmental factors. Possible explanations for negative D include: 1) changes in Δ canopy over decades as CO 2 and temperature have risen, and/or 2) post-photosynthetic fractionation processes leading to sequestration of isotopically-enriched carbon in long-lived pools like wood and soil.
Print ISSN:
0886-6236
Electronic ISSN:
1944-9224
Topics:
Biology
,
Chemistry and Pharmacology
,
Geography
,
Geosciences
,
Physics
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